1. Technical Field
The present invention relates to a technology for making a display element perform grayshade in such a manner that one field is divided into a plurality of sub-fields and then the display element is made into an on state or an off state in each of the sub-fields.
2. Related Art
When the grayshade is performed in an electro-optical device that uses display elements, such as liquid crystal elements, as pixels, the following technology has been proposed as a substitute for a voltage modulation method. That is, there has been proposed a technology for performing grayshade in such a manner that one field is divided into a plurality of sub-fields and then the pixels (liquid crystal elements) are made into an on state or an off state in each sub-field to thereby change the percentage of a time period during which the pixels are made into an on state or an off state within the one field, which is described in JP-A-2003-114661. Moreover, in the above technology, using the feature that the response speed of the liquid crystal elements is relatively slow, that is, specifically, using the feature that, even when the liquid crystal elements are made into an on state in only one sub-field, the transmittance ratio or reflectance ratio of each liquid crystal element does not reach black color that corresponds to an on state (is not saturated), the transmittance ratio or reflectance ratio of each liquid crystal element is minutely controlled in each of the sub-fields.
For example, in the above technology, fragmentation of gray scales from a basic m gray scale to a basic (m+1) gray scale is achieved in such a manner that, in the first sub-field, located on the preceding side of one field, to the (m+1)th sub-field in terms of time, sub-fields, in which the liquid crystal elements are made to enter an off state, are appropriately arranged (which is described in JP-A-2003-114661 and, specifically, shown in FIG. 16). More specifically, between the case in which the sub-fields, in which the liquid crystal elements are made to enter an off state, are arranged on the preceding side of one field in terms of time and the case in which the sub-fields, in which the liquid crystal elements are made to enter an off state, are arranged on the following side of the one field in terms of time, the transmittance ratio or reflectance ratio of the liquid crystal elements is closer to the saturation side in the latter case than in the former case. Thus, the brightness close to the basic (m+1) gray scale may be achieved.
Incidentally, the response speed of the liquid crystal generally increases as the temperature rises. Therefore, when the temperature of each display element is high and then the response speed has been increased, between the case in which the sub-fields, in which the liquid crystal elements are made to enter an off state, are arranged on the preceding side of one field in terms of time and the case in which the sub-fields, in which the liquid crystal elements are made to enter an off state, are arranged on the following side of the one field in terms of time, such an inconvenience that there is no difference in brightness of the display elements and thereby an appropriate gray scale display cannot be performed may be conceived. In addition, in the above technology, the scanning lines are selected in the order from the first line; however, in this configuration, the length of a time period of each sub-field needs to be set longer than time required to select all the scanning lines. Because the number of addressable luminance levels that can be displayed is determined by the length of a time period of the sub-field that is set the shortest, there is a problem that, when the length of a time period of each sub-field cannot be made short, the number of addressable luminance levels that can be displayed is limited.